Rohit,
Please see my comments in line.
Thanks
Jenny
On 7/25/2016 8:17 AM, Rohit Mohta wrote:
Hi All,
Can someone please help me understand the below snippet of Garbage
Collection logs?
(a)What’s the importance of values user, sys and real time values?
Should I care about those? For instance in the below snippet, ‘user’
has a value of 0.79second.
My understanding is,
·‘real’ represents the actual time taken by garbage collector process
to actually free up memory i.e. create object graph, and the whole
mark-sweep process.
Yes, it is the real time to free up the memory. It is the evacuation
time. It may/maynot include marking, depending on which gc you are using.
·‘sys’ represents the time spent in System Space (OS/Kernel related???)
yes
·‘user’ represents the time spent in User space i.e. application.
User time is the cpu time in user mode. If you have multiple cpus, and
use multiple gc threads, user time/real time ratio should be close to
thenumber of gc threads.
(b)I can see “Stopping threads took: 0.0001230 seconds” statement in
numerous occasions. Even when, GC was not taking place.
·What does it mean by ‘stopping threads took’? Does it mean, garbage
collector process was trying to ‘park’ all the threads, without
interrupting ongoing instructions. So it had to wait for ongoing
processing to complete, before it could put the thread in ‘park’ state?
For stop the world gc pauses, all application threads have to be
stopped. Stopping threads time is the time to bring the threads to a
safe point so that gc can process.
·Why does JVM stop application threads even when we don’t see any Full
GC events? In my system, full gc event is fired every 60 mins – but I
can see the above statement numerous times. Why does that happen?
Any stop the world pause need to do that. You might see those for minor gcs.
·When we talk about GC Suspension time, our definition is – “time for
which application will not process anything. Application kinda hangs”.
Should we consider these “Total time for which application threads
were stopped” also as Suspension time?
From the application point of view, yes.
·Any best practices to reduce the suspension time (apart from GC
friendly code)?
You can tune gc so that the stop-the-world time is minimal.
---------------------------------------------------------------------
2016-07-22T11:17:35.193+0000: 130991.925: Total time for which
application threads were stopped: 0.0014892 seconds, Stopping threads
took: 0.0001230 seconds
2016-07-22T11:18:05.192+0000: 131021.924: Application time: 29.9988249
seconds
2016-07-22T11:18:05.193+0000: 131021.925: Total time for which
application threads were stopped: 0.0010031 seconds, Stopping threads
took: 0.0001281 seconds
2016-07-22T11:18:12.845+0000: 131029.577: Application time: 7.6521209
seconds
2016-07-22T13:01:08.837+0000: 137205.569: [Full GC (System.gc())
[PSYoungGen: 9824K->0K(319488K)] [ParOldGen: 321122K->81024K(589824K)] 33
0946K->81024K(909312K), [Metaspace: 37949K->37949K(1083392K)],
0.1677301 secs] [Times: user=0.79 sys=0.00, real=0.17 secs]
Heap after GC invocations=23415 (full 41):
PSYoungGen total 319488K, used 0K [0x00000000e7000000,
0x00000000fbc00000, 0x0000000100000000)
eden space 299008K, 0% used
[0x00000000e7000000,0x00000000e7000000,0x00000000f9400000)
from space 20480K, 0% used
[0x00000000fa800000,0x00000000fa800000,0x00000000fbc00000)
to space 20480K, 0% used
[0x00000000f9400000,0x00000000f9400000,0x00000000fa800000)
ParOldGen total 589824K, used 81024K [0x00000000b5000000,
0x00000000d9000000, 0x00000000e7000000)
object space 589824K, 13% used
[0x00000000b5000000,0x00000000b9f20000,0x00000000d9000000)
Metaspace used 37949K, capacity 38526K, committed 38952K,
reserved 1083392K
class space used 3894K, capacity 4047K, committed 4144K, reserved
1048576K
}
2016-07-22T13:01:09.005+0000: 137205.737: Total time for which
application threads were stopped: 0.1749291 seconds, Stopping threads
took: 0.0001236 seconds
---------------------------------------------------------
Thank You
Rohit
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